Lambda regulation in combination with a catalytic converter is the most effective method today for purifying the exhaust from an internal combustion engine. Very low emission levels are achievable only by combination with the ignition and injection systems available today. Using a three-way or selective catalytic converter is particularly effective. This type of catalytic converter is capable of achieving greater than 98% degradation of hydrocarbons, carbon monoxide, and nitrogen oxides if the engine is operated at LAMBDA=1 in a range of approximately 1% around the stoichiometric air/fuel ratio. LAMBDA is an indicator of the deviation of the actual air/fuel mixture from LAMBDA=1, corresponding to a weight ratio of 14.7 kg/air to 1 kg/gasoline, as required theoretically for complete combustion, i.e., LAMBDA is the ratio of air mass supplied to theoretical air demand.
In lambda regulation, a measurement is performed on the particular exhaust and the quantity of fuel supplied is corrected immediately according to the results of this measurement via the injection system, for example. The measuring sensor is a lambda probe which shows a sudden change in voltage at precisely LAMBDA=1 and then delivers a signal indicating whether the mixture is richer or leaner than LAMBDA=1. The efficiency of the lambda probe is based on the principle of a galvanic oxygen concentration cell having a solid-state electrolyte.
Known lambda probes are two-point probes which operate by the Nernst principle using a Nernst cell. The solid-state electrolyte is composed of two interfaces separated by a ceramic. The ceramic material used becomes conducting for oxygen ions at approximately 350° C., so a Nernst voltage is generated when different oxygen levels prevail on the two sides of the ceramic between the interfaces. This voltage is a measure of the difference in oxygen levels on the two sides of the ceramic. Since the residual oxygen content of the exhaust of an internal combustion engine depends greatly on the air/fuel ratio the mixture supplied to the engine, it is possible to use the oxygen content of the exhaust as a measure of the actual prevailing air/fuel ratio.
With broadband probes, the measuring sensor is designed as a broadband sensor having solid electrolyte layers and a plurality of electrodes. Such a design is described in German Published Patent Application No. 199 12 102, in particular on pages 8 and 9 and in FIG. 1, to which reference is made to the full extent in the present context. These electrodes are also diagramed schematically in FIG. 1, which is described in detail below. A portion of these electrodes forms a pumping cell in this sensor, while another portion forms what is called a concentration cell. Furthermore, a first test gas space is formed by the solid electrolyte layers.
A pumping voltage U_APE,IPE is applied to the electrodes of the pumping cell (FIG. 1); a constant oxygen partial pressure is established in the first test gas space by pumping oxygen in or out using this pumping voltage. The pumping voltage here is regulated so that a constant voltage of 450 mV is established on the electrodes of the concentration cell. This voltage corresponds to a value of LAMBDA=1. Another electrode situated in a second test gas space is operated with one of these electrodes as the second pumping cell. Because of the catalytic material, this additional electrode functions as a NOx-sensitive electrode on which NOx is reduced according to the reaction NO→½N2+½O2. At the same time, the above-mentioned reference electrode functions as the second pump electrode at which the oxygen pumped out of the second test gas space is released into the atmosphere. A limit current is thus established on the electrochemical cell, which functions as an additional pumping cell, and this limit current is picked up as a test signal which indicates the NOx concentration.
It should be pointed out that the diffusion barrier described in German Published Patent Application No. 199 12 102 need not necessarily be included, and eliminating this barrier actually greatly reduces the gas travel times.
The measuring sensor described here may be used as a nitrogen oxide (NOx) sensor or as a hydrocarbon (HC) sensor. In its function as a NOx sensor, the NOx test signal reveals a dependence on the particular oxygen partial pressure in the measurement cell. This influence is due mainly to electric interactions of the sensor electrodes occurring in the sensor ceramic. The main influence is based on the main pump distance shown in FIG. 1 between the external pump electrode (APE) and the internal pump electrode (IPE), with the electric current (5 mA to 10 mA) and thus its pumping level having to be taken into account accordingly at a variable oxygen content.
This O2 influence is known to be compensated by electronic or computed addition or subtraction with an IPE current-dependent factor using suitable analyzer circuits, the gain of this compensation having to be set specifically for each individual sensor. Such a circuit for electronic compensation is shown in the block diagram in FIG. 1.
An object of the present invention is therefore to provide a method as defined in the preamble and a circuit which avoid the disadvantages mentioned above and minimize the above-mentioned influence of oxygen on the nitrogen oxide signal using the simplest possible technical means and the least expensive method possible.
The present invention is based on the idea of briefly suppressing the cause of the influence of oxygen on the nitrogen oxide signal, namely the main pumping current prevailing between the internal pump electrode (IPE) and the external pump electrode (APE), so that an uncorrupted NOx signal may be recorded during this period of time.
In a first variant according to the present invention, (main) pumping current I_pump is switched off, i.e., set at a value of 0, within a measurement time window T_mess. In a second variant, an IPE current control sets the main pumping current at a constant value>0 during measurement time window T_mess, so that although the influence of the main pumping current is not eliminated entirely, it is kept constant while the pumping level is reduced to a lesser extent and yet the amplitude of the oxygen concentration interference is reduced greatly.
In a preferred embodiment, measurement time window T_Mess is dimensioned so that the pumping current flowing between IPE and APE has already subsided within T_Mess and the increase in oxygen concentration due to the current switching off or current reduction has not yet reached the NOx electrode within T_Mess.
This above-mentioned intervention in the pumping current may be performed regularly at a repeat frequency, whereby the repeat frequency for the current switching off or current reduction is of such a dimension that the interference in the oxygen concentration has subsided again at the beginning of each subsequent IPE switching off or reduction. As an alternative, main pumping current I_pump may be switched off or reduced temporarily during operation of the nitrogen oxide sensor and a calibration performed then.
According to one exemplary embodiment, measurement time window T_Mess is in the range of 10-100 μsec, preferably 60 μsec, and the repeat frequency is in the range of 10-100 Hz, preferably 50 Hz.